Rigid polyurethane foams are recognized as excellent thermal insulation materials. Yet, they are not truly flame retardant. Isocyanurate foams are considerably more flame-retarding, but the unmodified foams made therefrom are highly cross-linked and are extremely brittle. Previous attempts have been made to reduce the brittleness of the isocyanurate foams by employing modifiers to reduce the amount of cross-linking. For example, flame-retardant urethane-modified polyisocyanurate rigid foams have been known since 1966 (Ashida, Polyisocyanurate Foams, Chap. 6, The Handbook of Polymeric Foams and Foam Technology, edited by D. Klempner and K. C. Frisch, Hauser Publishers, 1991, p 96); also amide-, carbodiimide- and imide-oxazolidone-modified polyisocyanurate foams. Ashida, ibid, p 97-8. Heat resistant and flame-retardant polyisocyanurate-polyurea foams prepared by reaction of diisocyanates with aqueous solutions of trimerization catalysts were disclosed in East German Patent 126,460. Urea-modified isocyanurate foams were disclosed in U.S. Pat. No. 4,425,446. The disclosed foams were useful in retrofitting wall cavities with insulating material, in that the material was substantially completely risen before setting or gelling. The urea-linkages were formed by an initial reaction between water and a multifunctional isocyanate which causes early rising of the foam. Additionally, the patentee discloses further modification of the formulation by the addition of a primary or secondary terminated polyamine, to form additional urea linkages. The polyisocyanurates of the present invention have rise times which exceed their gel times and set times.
Secondary amines have previously been proposed as curing agents for TDI-based flexible polyurethane foams, Gattuso et al Secondary Amine Extended Flexible Polyurethane-Urea Foams, Polyurethanes 88, Proceedings of the S.P.I. 31st Annual Technical/Marketing Conference, Oct. 19-21, 1988.
U.S. Pat. No. 3,846,351 describes the use of secondary phenylene diamines in combination with polyols as catalysts and chain extenders in the production of flexible polyurethane foams. More recently, it has been shown in U.S. Pat. No. 4,578,446 to House et al that N-alkylated methylenedianilines are suitable curing agents for urethane prepolymers, i.e., in elastomer production via non-RIM processes. In U.S. Pat. No. 4,801,674 to Scott et al, N-alkylated methylene dianilines are disclosed as suitable curing agents for RIM applications. However, neither patent discloses the unexpected beneficial results achieved with the rigid polyisocyanurate foams of the present invention.